The invention relates generally to magnetic resonance imaging systems (MRI) and more specifically to switching amplifiers adapted for use in MRI systems.
A conventional MRI device establishes a homogenous magnetic field generally along a central axis of a subject that is to undergo an MRI procedure. This homogeneous magnetic field affects gyro magnetic material of the subject for imaging by aligning the nuclear spins (in atoms and molecules forming the body tissue in medical applications, for example) along the direction of the magnetic field. If the orientation of the nuclear spins is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their spins with the field. Perturbation of the orientation of nuclear spins is typically caused by application of radio frequency (RF) pulses tuned to the Larmor frequency of the material of interest. During the realignment process, the nuclei precess about their axes of and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or about the subject.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the location of the nucleus. It is possible to distinguish signals originating from different locations within the subject by using encoding, typically phase and frequency encoding, created by gradient coils that apply gradient fields over the main magnetic field. A typical MRI system comprises three gradient coils for providing respective fields along the X, Y and Z axes. Control of the gradient coils allows for orientation of the axes for encoding of locations within the subject, and for selection of a desired “slice” for imaging.
Gradient coils produce additional magnetic fields that are superimposed on the primary magnetic field to permit localization of the image slices and also provide phase encoding and frequency encoding. The encoding permits identification of the origin of resonance signals during later image reconstruction. The image quality and resolution depends significantly on how the applied fields can be controlled. To achieve faster imaging rates, the gradient fields are typically modified at frequencies of several kHz. Control of the gradient coils is generally performed in accordance with pre-established protocols or sequences, called pulse sequence descriptions, permitting many types of tissues to be imaged and distinguished from other tissues in a medical context, or for imaging various features of interest in other applications.
Typically, a gradient coil operates in ranges of several hundred amperes of current and several thousand volts. Therefore, the gradient coil requires a gradient amplifier to supply the coils with the required current and voltage levels. The gradient amplifier is typically a power amplifier. Amplifier designs may be quite complex because the required precision in signals may be less than a milliamp for any current level in the coil current for high performance MRI systems providing quality images.
Earlier implementations of gradient amplifiers used linear amplifiers that provided high fidelity. However, given present power levels, the use of these amplifiers becomes impractical due to the higher voltages and currents. Present day techniques use hybrid systems that combine linear amplifiers with switching power stages. Such systems use bridges in parallel or bridges stacked to meet the system requirements, typically employing power semiconductor devices. The linear amplifiers provide bandwidth and control, while the switching power stages provide a boost in voltage for fast transitions.
While there are inherent advantages of such systems, such as the ability to provide higher power levels, the performance of these hybrid designs is limited by the capabilities of the linear amplifiers. A better functioning gradient coil, capable of inducing better gradients in the primary magnetic field could result in numerous advantages, such as increase in the cost effectiveness, improvement of the dynamic performance, decrease in the examination time, improvement in spatial resolution, improvement in temporal resolution and significant improvement in the quality of the resultant images.
In view of the limitations of current MRI gradient amplifier techniques, there is a need for a new power stage architecture that provides high power and delivers high fidelity through novel circuit topologies and control mechanisms.